>
>On Mon, 7 Jul 97 22:30:17 -0700 Rich Measures <measures@vc.net> writes:
>>>> From: Rich Measures <measures@vc.net>
>>>>
>>>> For adding a step-start relay and high speed switching, a FWB is
>>>> indicated. Half-wave rectification causes DC to unhappily flow in
>>the
>>>> transformer. Adding 3 more diodes and ungrounding the 80v winding
>>is
>>>> easy/inexpensive.
>>>
>>>The very small power demand of the bias supply (about 12 watts)
>>>would not greatly affect the core losses of a transformer already
>>>delivering 160 watts to a resistive load.
>>>The half wave rectifier is perfectly fine.
>>
>>- 20uF in half-wave service is perfectly fine for filtering the
>>original
>>20mA load, however, it is a bit lacking for filtering the 100mA that
>>the
>>high speed switching and step-start circuits consume. Increasing C is
>>
>>one fix. Doubling the ripple frequency, by converting to FWB
>>rectification is another fix. 3 rectifiers cost less than another
>>filter
>>C.
>
>True, but the original discussion was about the capacitor failing and
>requiring replacement anyway. The cost of a 100-200MF 160V capacitor, if
>someone wants to add a bunch of extra parts will not require a 2nd
>mortgage. The 90V winding on the xfmr is rated 100ma CCS.
>
With half-wave rectification, a 100mA winding is not good for 100mA DC.
With full-wave bridge rectification, it pretty much is.
>>
>>>If I was going to do a mod, I'd get rid of the whole 130 volt supply,
>>
>>>and let the tube self-bias off. Than a more conventional 12 volt
>>>relay could be used.
>>>
>>Self cut-off biasing is good. However, conventional relays are too
>>slow
>>to keep pace with modern radios.
>
>QSK operators are a small minority; the original relay is fine for the
>rest of us.
We are not talking about QSK operators, Carl. A 5mS QSK radio starts
delivering RF in 5mS---even on push-to-talk. A 15 to 25 mS amplifier
(SB-220, TL-922, L4B, 2K-4, et cetera) can not switch fast enough to keep
up with modern radios. The result is hotswitching the amplifier's relay.
The solution is modifying the amplifier for high speed switching. The
total cost is about $65. An added bonus is QUIETNESS. (circuit in 1/94
QST). We sell a parts kit for $20 that is missing the Jennings RJ-1A
vacuum relay, which can be obtained from Fair Radio.
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|